Semiconductor lasers used for an active layer having quantum dots have been developed in recent years. Patent Document 1 discloses a method for forming quantum dots. Patent Document 2 discloses a semiconductor laser in which at least part of an active layer including a plurality of quantum dots is formed of p-type semiconductor in order to achieve high-speed modulation and high reliability.
Referring to FIG. 1E of Patent Document 2, a p-type clad layer 2 and a quantum dot active layer 3 are stacked on a p-type GaAs substrate 1. An n-type clad layer 4 having a convex shape (ridge portion) is provided on the quantum dot active layer 3. The n-type clad layer 4 is formed only at a central portion of the quantum dot active layer 3. Thus, an area of p-n junction formed by the quantum dot active layer 3 and the n-type clad layer 4 can be reduced. Therefore, the parasitic capacitance can be reduced, so that high-speed modulation can be achieved.    Patent Document 1: Japanese patent No. 3468866    Patent Document 2: Japanese laid-open patent publication No. 2006-286902